Rapid advances are occurring in various electronic devices especially display devices that are used for various communicational, financial, and archival purposes. For such uses as touch screen panels, electrochromic devices, light emitting diodes, field effect transistors, and liquid crystal displays, conductive films are essential and considerable efforts are being made in the industry to improve the properties of those conductive films.
There is a particular need to provide touch screen displays and devices that contain improved conductive film elements. Currently, touch screen displays use Indium Tin Oxide (ITO) coatings to create arrays of capacitive areas used to distinguish multiple point contacts. ITO coatings have significant short comings. Indium is an expensive rare earth metal and is available in limited supply from very few sources in the world. ITO conductivity is relatively low and requires short line lengths to achieve adequate response rates. Touch screens for large displays are broken up into smaller segments to reduce the conductive line length to an acceptable resistance. These smaller segments require additional driving and sensing electronics. In addition ITO is a ceramic material, is not readily bent or flexed, and requires vacuum deposition with high processing temperatures to prepare the conductive layers.
Silver is an ideal conductor having conductivity 50 to 100 times greater than ITO. Silver is used in many commercial applications and is available from numerous sources. It is highly desirable to make conductive film elements using silver as the source of conductivity.
Numerous publications describe the preparation of electrically-conductive films formed by reducing a silver halide image in silver halide emulsions in the form of electrically-conductive grid networks having silver wires having sizes of less than 10 μm. Various efforts have been made to design the silver halide emulsions and processing conditions to optimize such electrically-conductive grid networks and the methods for making them.
For example, improvements have been proposed for providing electrically-conductive grid patterns from silver halides by optimizing the silver halide emulsions as well as finding optimized processing solutions and conditions to convert latent silver images into silver metal grid patterns. The precursors used to provide the conductive films can comprise one or more silver halide emulsion layers on opposing sides of a transparent substrate, along with optional filter layers and hydrophilic overcoats.
While these processes and articles can provide desired electrically-conductive films, optimizing the design of both the precursors and processing procedures requires considerable effort in order to achieve the exacting features required in electrically-conductive films to be incorporated into touch screen displays.
Other industrial approaches to preparing electrically-conductive films or elements have been directed to formulating and applying photocurable compositions containing dispersions of metal particles such as silver metal particles to substrates, followed by curing of the photocurable components in the photocurable compositions. The applied silver particles thus act as catalytic (seed) particles for electrolessly plated electrically-conductive metals. Useful electrically-conductive grids prepared in this manner are described for example in WO 2013/063183 (Petcavich), WO 2013/169345 (Ramakrishnan et al.). Other details of a useful manufacturing system for preparing conductive articles especially in a roll-to-roll manner are provided in PCT/US/062366 (filed Oct. 29, 2012 by Petcavich and Jin).
Using these methods, photocurable compositions containing silver particles can be printed and cured on a suitable transparent substrate for example a continuous roll of a transparent polyester, and then electroless plating can be carried out. These methods require that high quantities of silver particles be dispersed within the photocurable compositions in a uniform manner so that coatings or printed patterns have sufficiently high concentration of catalytic sites. This generally cannot be achieved without carefully designed dispersants and dispersing procedures and such dispersants can be expensive and hard to use in a manner to provide reproducible products in a high speed manufacturing operation. Without effective dispersing, silver particles can readily agglomerate, leading to less effective and uniform application of catalytic metal patterns and electroless plating. It is therefore difficult to provide uniform electrically-conductive films having the desired electrically-conductive metal patterns.
U.S. Patent Application Publication 2007/0261595 (Johnson et al.) describes a method for electroless deposition on a substrate that uses an ink composition containing silver as a reducible silver salt and filler particles. After reducing the reducible silver ions, such compositions can be cured for improved adhesion to the substrate especially if the compositions contain an UV-curable monomer or oligomer.
U.S. Pat. No. 7,875,416 (Park et al.) describes photosensitive compositions comprising a multifunctional epoxy resin, a photoacid generator, an organic solvent, and silver particles.
A common coordinating ion to form organic silver complexes is carboxylic acid [Prog. Inorg. Chem., 10, 233 (1968)]. However, silver-carboxylate complexes are generally sensitive to light and hardly soluble in organic solvents [U.S. Pat. No. 5,491,059 of Whitcomb and U.S. Pat. No. 5,534,312 of Hill et al.] and have a high decomposition temperature. Thus, such complexes have little utility in spite of ready availability. To solve this problem, several methods have been proposed for example, in Ang. Chem., Int. Ed. Engl., 31, p. 770 (1992), Chem. Vapor Deposition, 7, 111 (2001), Chem. Mater., 16, 2021 (2004), and U.S. Pat. No. 5,705,661 (Iwakura et al.). Among such methods are those using carboxylic acid compounds having long alkyl chains or including amine compounds or phosphine compounds. However, the silver derivatives known thus far are limited and have insufficient stability or solubility. Moreover, they have a high decomposition temperature needed to be useful for pattern formation and are decomposed slowly.
U.S. Pat. No. 7,682,774 (Kim et al.) describes other photosensitive compositions comprising silver fluoride organic complex precursors as catalyst precursors. This patent describes the use of polymer derived from a monomer having a carboxyl group and a co-polymerizable monomer that may provide polymeric stability and developability of the resulting “seed” silver catalyst particles used for electroless plating.
Despite the various techniques described in the art, there remains a need for an improved means for providing silver catalytic (seed) particles for electroless plating in the formation of electrically-conductive patterns in a reproducible and high-speed manner suitable for continuous high-speed production. It is particularly desirable to provide a method for high-speed production of electrically-conductive patterns without the need for complicated and uncertain dispersing procedures for high concentrations of silver metal particles.